Ion implanters are commonly used in the production of integrated circuits (IC) and flat panels displays to create in a semiconductor wafer, usually silicon, regions of different conductivity by p- or n-type doping. In such devices, a plasma source is used to ionize the dopant gas. A beam of positive ions is extracted from the source, accelerated to the desired energy, mass filtered and then directed toward the wafer. As the ions strike the wafer, they penetrate to a certain depth (depending on their kinetic energy and mass) and create regions of different electrical conductivity (depending on the dopant element concentration) into the wafer. The n- or p-doping nature of these regions, along with their geometrical configuration on the wafer, define their functionality, e.g., n-p-n or p-n-p junctions within the transistors. Through interconnection of many such doped regions, the wafers can be transformed into complex integrated circuits.
A block diagram of a representative ion implanter 50 is shown in FIG. 1. Power supply 1 delivers the required energy (DC or RF) to the plasma source 2 to enable ionization of the doping gas. The gas is fed into the plasma chamber through a mass-flow controlled system (not shown) under the pressure in the mTorr range, ensured by a vacuum pumping system. Depending on the desired dopant, different fluoride or hydride doping gases, such BF3, PF3, AsF3, GeF4, B2H6, PH3, AsH3, GeH4 or others, with or without co-carrier gas, are introduced. The plasma chamber has an aperture 3 through which the ions are extracted by a combination of electrodes 4. A commonly used scheme is a triode combination in which the plasma chamber aperture is at high positive potential, then a second electrode (suppression electrode) at negative potential and finally a third electrode at ground potential. The role of second electrode is to prevent secondary electrons from streaming back to the plasma chamber. However, other extraction electrode combinations such as thetrode, pentode or Eisel lenses, are also possible. These exiting ions are formed into a beam 20, which then passes through a mass analyzer 6. The extracted ion beam is composed of a mixture of ions. For instance, the beam extracted from BF3 gas will be comprised mainly of BF3+, BF2+, BF+, B+, and F+ ions. Therefore, it is necessary to use the mass analyzer to remove unwanted components from the ion beam, resulting in an ion beam having the desired energy and composed of a single ionic specie (in the case of BF3, the B+ ion). To reduce the energy to the desired level, ions of the desired species then pass through a deceleration stage 8, which may include one or more electrodes. The output of the deceleration stage is a diverging ion beam. A corrector magnet 10 is used to expand the ion beam and then transform it into a parallel ribbon ion beam. Following the angle corrector 10, the ribbon beam is targeted toward the wafer or workpiece. In some embodiments, a second deceleration stage 12 may be added. The wafer or workpiece is attached to a wafer support 14. The wafer support 14 provides a vertical motion so that the wafer can be brought in the beam path and then passed up and down through the fixed ion ribbon beam. It also can be rotated so that implants can be performed at different incidence angles with respect the wafer surface. With the wafer out of the beam path, the beam current can be measured by a Faraday cup 16. Based on the beam current value and the desired dose, the wafer exposure time or the number of passes through the ribbon ion beam is calculated.
Taking into account that the rate of ion extraction from the plasma source is given bydNextr/dt≅AnvB where A is the area of the extraction aperture, n the plasma density, and vB=(kBTe/mi)1/2 the Bohm velocity (with kB, Te and mi the Boltzmann constant, electron temperature and ion mass, respectively) a limited number of plasma sources have proved to have sufficient plasma density to be useful as ion sources. In some embodiments, such as Barnas sources, an arc discharge creates the plasma. Tungsten filaments are used to generate a flux of electrons needed to sustain the high arc plasma density. In other embodiments, such as indirectly heated cathodes (IHC) which are also a form of arc discharge, to prevent the filament from detrimental exposure to the plasma and therefore to extend the lifetime of the source, the necessary electrons are provided by thermionic emission from an indirectly heated cathode. While these thermal plasma sources are effective in generating the desired ions, they are typically only used to create atomic ions, due to the high temperatures developed within the arc chamber. Because dissociation energies are typically low, the thermal energy in the arc plasma is often high enough to breakdown molecular bonds and to fractionate the feeding gas into smaller molecules or atoms.
It has been found that for shallow implants applications where low ion energy is required, in order to overcome the detrimental space-charge effects and to increase the productivity of the ion implantation process, molecular gases with higher content of the active dopant in the molecule such as C2B10H12, B10H14, and B18H22 can be used. The resulting molecular ions can be accelerated at higher energies, thus preventing the beam from the space-charge detrimental effects. However, due to their heavier mass, shallow implants can be performed. For such implantation processes that require molecular ions rich in active dopant rather than dopant atomic ions, low temperature plasma sources such as RF inductively coupled discharges are well suited. In these discharges, the plasma is produced by coupling the power from an RF generator through an antenna. The high RF currents flowing through the antenna give rise to an oscillatory magnetic field which, according to the Maxwell's 3rd electrodynamics law:∇×{right arrow over (E)}=−∂{right arrow over (B)}/∂t produces intense electric fields in a limited spatial region (skin depth) which is a function of the RF excitation frequency and gas pressure. Electrons accelerated by these electric fields gain enough energy to ionize the gas molecules and create a plasma. The created plasma is not in thermal equilibrium since electrons have a temperature (usually ˜2-7 eV) much higher than ion or neutral temperature (usually slightly above the room temperature). While this discharge is useful in the generation of molecular ions, its efficiency is often less than desired since the plasma density is ≦1011 cm−3, which is about one to two orders of magnitude less than arc discharge.
Another potential plasma source for ion implantation purposes is helicon discharge, which is able to generate high plasma densities at relatively low gas temperatures. Different than other RF plasma sources, in helicon discharges, electron heating is based on collisional damping of helicon waves. These waves, which are a particular case of whistler waves, are excited by an RF antenna immersed in a DC magnetic field. The low pressure working gas is introduced in a dielectric chamber, usually a quartz or Pyrex cylinder and the antenna is wrapped around it. Electrons gain energy from the wave and, if their energy is above ionization threshold energy, new electron-ion pairs are created through ionization collisions with the neutral gas atoms or molecules. After each ionization event, this wave can quickly bring electrons to the optimum energy for another ionization process. Furthermore, besides governing the helicon wave excitation, magnetic field presence ensures a plasma confinement, thus reducing the loss of charged particles to the walls of the chamber. It was believed that high ionization efficiency of helicon sources might also come from Landau damping (the resonant damping occurring when the phase velocity of the helicon wave is closer to the electron velocity at energies corresponding to the peak in the gas ionization cross-section). However, the experiments showed that Landau damping accounts for only a few percent of the total energy transferred to electrons. Another energy transfer mechanism that can be accounted for high ionization efficiency of helicon discharge consists of excitation of an electron-cyclotron wave near the surface of the chamber wall, the Trivelpiece-Gould mode, followed by a rapid damping. Another possible mechanism consists in nonlinear or parametric coupling of helicon waves to ion-acoustic or lower-hybrid waves followed by their rapid damping. Even not yet elucidated, the energy deposition mechanisms in helicon discharge are very efficient, thus giving rise to high ionization efficiency and therefore, plasma density is usually from one to three orders of magnitude higher than in other RF plasma sources, such as capacitively (CCP) or inductively coupled (ICP) discharges, for a given input power. As compared to electron-cyclotron resonance plasma sources (ECR), which are comparable from the plasma density point of view, helicons have the advantage of running at lower magnetic fields, i.e., 200-300 Gauss compared with 875 Gauss which is necessary for a 2.45 GHz ECR source and higher for higher frequencies.
The afore presented characteristics of the helicon plasma source make it an attractive option as a molecular ion source for ion implantation. Although invented almost four decades ago, helicon discharge has been developed for industrial applications only in the last 10-15 years. Mostly, its application in industry dealt with plasma etching and plasma deposition in the semiconductor industry. However, as it is pointed out below, to date, helicons have not been effectively used as part of an industrial ion source, mainly due to their non-uniform plasma density distribution.
When running in helicon mode the plasma column has a very bright central core denoting a very peaked density profile on the axis of the discharge. Therefore, typically a diffusion chamber is used in conjunction with the source so that the plasma generated in the helicon source expands within the chamber and the peaked density profile relaxes. As FIG. 2 shows, although smoother, the density profile still tracks the profile in the source: a relatively high plasma density on the axis of the discharge but a significant decrease in density along the radial direction. Furthermore, an increase in power only serves to accentuate this characteristic, i.e., by increasing the density at the peak, while narrowing its uniform density radial range. This non-uniform plasma density profile along radial direction characteristic limits the application of this source for large area plasma processing.
In the prior art, there have been attempts to overcome this main drawback of the helicon created plasma, i.e., radial plasma density non-uniformity. To date, helicons have been used in plasma etching and plasma deposition and, to a lesser extent, in ion beam generation. Specifically, helicons have not been used in generating of ribbon ion beams typically used in ion implanters. Therefore, an ion source that can effectively utilize the high plasma density produced by the helicon source and create a wide and uniform ribbon ion beam would be beneficial from ion implantation perspective.